Advances in cellular radio-telephony have led to a hybrid analog/digital radio-telephone. A transmitter of such a telephone converts digital signals containing control and message information into analog signals for communication. More specifically, the transmitter forms transmission signals by modulating analog carriers with encoded versions of the digital signals. A common encoding scheme is phase-shift keying in which the digital signals are differentially encoded as changes in phase in accordance with an encoding algorithm.
A known modulation technique is quadrature modulation, which entails modulating two orthogonally-related sub-carriers (i.e., two analog signals that are 90 degrees apart in phase) with the encoded data. Typically, in quadrature modulation, the digital signals are converted into two parallel bit streams, and encoded as described above. Then, one of the encoded bit streams modulates a first of the sub-carriers, and the other encoded bit stream modulates a second of the sub-carriers. Subsequently, the modulated sub-carriers are added for transmission. The modulated sub-carriers are called the transmit in-phase ("I") signal, and the transmit quadrature ("Q") signals.
For recovery of digital data from received encoded, quadrature-modulated signals, a radio-telephone receiver employs a quadrature demodulator. For instance, the quadrature demodulator has a pair of mixers, each of which multiplies the received signal with one of two, different signals generated by a local oscillator and having orthogonally related phases, thereby producing baseband signals. The baseband signals are subsequently converted into digital signals and processed (e.g., filtered) along separate circuit paths, called respectively the "I" and "Q" channels.
The resulting signals, i.e., the RECEIVE.sub.-- I and RECEIVE.sub.-- Q signals, are then decoded to data in a decoder using, essentially, the reverse of the encoding algorithm. Ideally, RECEIVE.sub.-- I and RECEIVE.sub.-- Q are identical to the corresponding encoded bit streams produced by the encoders in the transmitter, in which case the receiver can recover the data accurately. In other words, the receiver can exhibit a "data recovery rate" of 100%. The data recovery rate is the number of correctly identified or recovered bits in a digital signal of preselected length divided by the total number of bits in that signal.
While such a receiver appears generally suitable for its intended purposes, its data recovery accuracy will depend on the extent to which the RECEIVE.sub.-- I and RECEIVE.sub.-- Q signals as supplied to the decoder are corrupted due to phase and/or amplitude distortion. Distortion in these signals can result in data errors: the more extensive the distortion, the lower the data recovery rate.
The distortion causes components of the RECEIVE.sub.-- I signal to appear in the RECEIVE.sub.-- Q signal, and components of the RECEIVE.sub.-- Q signal appearing in the RECEIVE.sub.-- I signal. These cross-over components are called "cross-talk." Unfortunately, decoding of signals corrupted with cross-talk can, and often will, result in data recovery errors, and performance degradation ultimately in the receiver.
Cross-talk-producing distortion can originate, for example, either during transmission or within the receiver itself. In cellular radio-telephony, for instance, transmission-originated distortion is a propagation effect arising while the communication signal is traversing the air-waves, e.g., due to multi-path fading.
Receiver-originated distortion is caused typically by various combinations of contributing factors, many of which are inherent in electronic devices and signal processing, and often are not readily controllable. For instance, potential sources of such distortion are receiver components having non-linear transfer functions (e.g., amplifiers, mixers, and limiters), signal-synchronization errors, impedance mismatch, filter-center-frequency offsets, electronic-device bandwidth tolerances, and oscillator-frequency drift.